This invention relates to a system for accurately and quickly diluting and mixing fluid samples using a 3-way valve and pulse width modulation, such that the proportion of each fluid in the mixture can be accurately known. In particular, this invention relates to a system for delivering such fluid samples to a chemical analysis instrument.
Dilution is the act of taking a concentrated fluid and making it less concentrated by mixing it with another, different fluid (a diluent). The diluent is usually a fluid whose composition, concentration, and other physical and chemical properties are all well known. A common diluent is water, H2O. Ideally, there are no chemical or physical reactions between the diluent and concentrated fluid when mixed. When diluting a concentrated fluid, it is essential to know the degree to which the original fluid is diluted in order to be able to relate the diluted fluid to the original concentrated fluid. For example, it""s critical to a chemical analysis to know whether the original concentrated fluid was diluted to 5% or to 95% of its original concentration.
Conventionally, mixing and dilution systems utilize pipettes or pumps (or some other means of moving fluids) to deliver different fluids to a mixing junction. To mix two fluids, the fluids are delivered to a T-junction or Y-junction (through separate input channels) where they are mixed and then exit the junction through a single output channel. These types of systems use linear power delivery to control the amount of fluid that is allowed to enter the mixing junction through each input channel. A problem with these linear control devices is that the efficiency of the electronic control components is poor when operated at mid-range power levels, and power is lost as heat in the components of the electrical circuits controlling the device. More importantly, the linear power delivery is not accurate enough for high-precision mixing applications when very small volumes are involved.
U.S. Pat. No. 5,646,727 (July 1997), issued to Hammer et al., is a specific example of a linear method of mixing and diluting. Hammer et al use peristaltic pumps to deliver sample solutions to a 3-way T-junction or a 4-way X-junction. With the T-junction setup, there is a sample solution connected to an input channel and a peristaltic pump is used to deliver the fluid to the junction. There is a diluent solution connected to the other input channel (with no pump) and the output channel is connected to a nebulizer of a spectroscopic instrument. With the X-junction setup, there is an additional input channel to which a standard solution is connected with a peristaltic pump for fluid delivery to the junction. The nebulizer operates by aspiration and so there is always a drop in pressure between the junction and the nebulizer. Under proper operating conditions, this vacuum pressure can be used to draw up diluent and pull it through the junction. The amount of sample or standard delivered to the junction depends on the pumping speed of the peristaltic pumps. The electrical power supplied to these pumps is delivered by the linear method described above.
Just as traditional power delivery methods are called xe2x80x9clinearxe2x80x9d, traditional methods of mixing and diluting fluids can also be called xe2x80x9clinearxe2x80x9d. Traditional methods use a T-junction to mix and dilute fluids, and allow both of the two input fluids to flow freely and unimpeded into the junction at the same time. That is, the junction in which the fluid mixing occurs has no valves to control the flow of fluids into it. Traditional methods rely on the control of the flow rates of the two input fluids. In this way, control of how much of each fluid enters the junction per unit time is achieved, and therefore control the degree of mixing or diluting is achieved. This method can be considered xe2x80x9clinearxe2x80x9d because the controlling method is the linear control of the flow rates of the input fluids (i.e. the flow rates of the pump are adjustable anywhere between LOW and HIGH rates).
Traditionally, 3-way valves have the ability to select which input fluid path flows through the valve (i.e.ON/OFF control). However, presently there is a need for a 3-way valve that may be used for the purpose of mixing and/or diluting fluid samples, in addition to basic fluid flow selection. As a result, it is an object of the present invention to use a 3-way valve for the purpose of mixing and/or diluting fluid samples.
It is a further object of the present invention to provide a system that integrates the pulse width modulation (PWM) method of power delivery with a 3-way valve apparatus for delivering a diluted sample (of varying degrees of dilution) to a sample introduction apparatus of a chemical analysis instrument, along with methods for on-line calibration and sample analysis.
The present invention is a 3-way valve with two inlet ports, and an outlet port. The 3-way valve controls which inlet port has an open path to the outlet port via a switchable gate located inside the junction of the three ports. The gate is controlled by an electrical input into a solenoid coil, such as the 3-way solenoid valve provided by Lee Co. (model # LHDA 0523112H), which enables the gate to be switched between the two inlet ports to provide an open path for one or the other, but never both at the same time. Two different fluids are connected to the inlets and introduced into the valve. The fluids are introduced into the valve either by suction from the outlet port or by pressure applied on the inlet fluid reservoirs, the electrical input is by pulse width modulation (PWM).
The flow of a fluid through the 3-way valve junction is either ON or OFF. The free and unimpeded flow of both fluids (simultaneously) into the outlet port is not allowed. Fluids are only allowed to flow separately into the outlet port (one at a time). If one fluid is flowing into the outlet port (it is ON) then it is flowing in at a constant flow rate and the other fluid is not flowing into the outlet port at all (it is OFF). There is no intermediate state between ON and OFF, effectively resulting in a digital system.
The dilution of a concentrated solution is achieved is by feeding the concentrated sample solution into one inlet port (the sample path) and feeding a diluent into the other inlet port (the diluent path). One of the two inlet ports is opened to allow fluid to flow through the valve junction and into the outlet port, while the gate is rapidly switched ON and OFF between the two ports. Over a period of time where many ON/OFF cycles are completed, the flow of a fluid through the 3-way valve takes on a xe2x80x9cpulsedxe2x80x9d nature. The main controlling factors that affect dilution using this method are the frequency with which the inlet paths are switched, the duration of each ON and OFF period for each inlet fluid, and the constant flow rate (or pressure) of each inlet fluid.
This system is capable of passing gaseous or liquid samples through the 3-way valve, and the degree of dilution can vary between 0-100%. The invention is applicable to any chemical analysis instrument or measuring technique that can facilitate a liquid or gaseous sample.